Mass spectrometry



Dec. 3l, 1946. H. w. wAsHBuRN 2,413,668

` MASS SPECTROMETRY Filed Nov. 2'1, 1944 A TTORNE' YS HA/Poco M M45/manuPatented Dec. 31, 1946 MASS SPECTROBIE'IRY Harold W. Washburn, Pasadena,Calif., assigner to Consolidated Engineering Corporation, Pasadena,Calif., a corporation of California- Application November 21, 1944,Serial No. 564,425

Claims. (C1. 'i3-18) 1 This invention is concerned withmass spectrometryand particularly with the analysis of mixtures with a mass spectrometer.It provides a novel head or ionizing apparatus for a mass spectrometer.

The mass spectrometer is adapted for use in both quantitative andqualitative analyses. It has been employed for scientiiic studies suchas the measurement of isotope ratios and more recently for other worksuch as the analysis of complex mixtures .of hydrocarbons. It isessentially an apparatus for producing ions and sorting'them accordingto the ratio of their mass 'to their charge, i. e. according to their'specic mass. `A sample to be analyzed, for example a lgas mixture, isionized in a chamber preferably by electron bombardment and theresulting ions are propelled by an electrical field through an aperture'into an analyzer. There the ions under the influence of a magnetic orelectric field are sorted according to their specific mass, ions of lowspecic mass pursuing a different path than ions of high specific mass.Assorted ions are collected and discharged the quantity of each kind ofions being measured by the amount of current that they discharge uponcollection.

As a result of my investigations, I have developed an improved apparatusfor converting molecules of a sample to be analyzed into ions andpropelling and collimating the ions thus formed into a beam which may bepropelled into an analyzer and there separated into a plurality ofdivergent beams under the influence of a magnetic or electrical iield.The ionizing chamber or head of my apparatus may be employed with avariety of types of mass spectromet'ers, in which the head itself isoutside the main magetic Held. Thus, it may be usedv to great advantagewith the form of mass spectrometer in which the head is outside f themagnetic eld which is employed to separate the ions of the beam into aplurality of divergent beams. This type of instrument is illustrated atpage 213, of Review of Scientific Instruments, vol. 11, 1940.

In heretofore customary types of mass spectrometers, ionization ofmolecules of a sample has been carried out for example, by electronbombardment in the space between a pusher electrode independentlyenergized and a second electrode which ordinarily has been provided Witha slit through which the ions arepropelled as a result of `a potentialdeveloped between the pusher electrode and the slit electrode. I havefound that it is possible to construct a mass spectrometer .of simpledesign in which no pusherelectrode per se .is provided. I'hus myinvention.

borhood of the inlet thereto being of conductive material, means forprojecting an electron beam across the gap transverse to the axis of theinlet of the second chamber and the outlet of the first chamber incontact with molecules passing out of that outlet, an electrode disposedin the second I maSSES.

chamber, and means for establishing-an ion propelling potential in thesecond chamber between the inlet of the second chamber and theelectrode.

In this type of apparatus, there is no pusher electrode. Instead, theions formed by electron bombardment in 'the neighborhood of the beam areattracted into and propelled through the second chamber by a negativepotential established at the electrode disposed in the second chamber,for example, at its outlet. With this arrangement it is unnecessary toprovide a magnetic eld in the ionization chamber since the electron beamtravels only across the narrow slit or gap. This permits simplicity ofconstruction. The structure of the invention permits simultaneousrecording of a wide range of specic Moreover, it minimizes surfaceeffects,- since no ions collide with collimating slits unless they arehighly energized and so relatively immune to small changes in surfacepotential.

It is desir-able to provide auxiliary, electrodes within the, secondchamber along the path of the beam therethrough. These electrodes may,but need not. be maintained increasingly negative in the direction ofpassage of the'electron beam and the voltage applied on them should besuch that the beam is brought to a focus at some point along its path oftravel.` s l In the preferred form of my apparatus, I provide at leastoneauxiliary electrode mounted to one side of the beam path, means beingprovided for impressing a potential on that electrode to aiect thefocussing of the beam. Preferablyj provide at least one set of auxiliaryelectrode?,

vthe members of the set being mounted oppositey two chambers to anelectron catcher I1.

potential difference between the two electrodes may be employed toaffect the focussing of the beam. If desired, a plurality of such setsof auxiliary electrodes may be placed along the beam path within thesecond chamber. For focussing purposes, it is desirable that lines drawnbetween the electrodes of each set make different angles with the pathofthe beam.

These and other features of my invention will be more thoroughly.understood in the light of the accompanying single gure which is aschematic diagram of a head for a mass spectrometer constructed inaccordance with a preferred modification of my invention.

Referring to the drawing, it will be observed that theapparatuscomprises a first chamber I provided with a gas inlet II and aconstricted gas outlet I2. Preferably this chamber has a metal wall, butin any event that portion of its wall adjacent the outlet should be ofconductive metal to facilitate leakage from the electrical field intothe portion of that chamber adjacent the outlet. thereby aiding in thepropulsion of ions from the first chamber I0 through the apparatus. Assliown in the drawing, t-he rst chamber preferably tapers to the outletwhich may be in the form of a narrow slit. Adjacent the first chamberI0, but separated from it by a gap is a second chamber I3. This chamberhas an inlet I4 adjacent the outlet of the first chamber and in linewith it so that both apertures are coaxial with the path of the ionbeam. An electron gun I5 is provided at one side of the gap separatingthe two chambers. An electron beam I6 is propelled from the gun andpasses through the gap between the The electron gun `and catcher may beof conventional construction, for example of the construction shown inmy copending application Serial No. 513.528, ledDecember 9, 1943.

The upper end of the second chamber is in the form oi an electrode I8 felectrical conductive material and is provided as indicated hereinbeforewith a slit Si which matchesthat at the outlet of the first chamber.Similarly the outlet end of the second chamber (the lower end as shownin the drawing) is provided with an electrically Conductive electrode I9provided with a wide slit S2 through which the ion beam passes into ananalyzer (not shown). Within the second chamber, which for the most partis walled with nonconductive material 20, are disposed a rst pair offocussing electrodes 22, 23 and a second pair of focussing electrodes25, 26. In the type of apparatus illustrated, the outlet I2 of the firstchamber of the inlet S1 and the outlet S2 of the second chamber areinthe form of slits of elongated cross-section, the long dimension beingat right angles to the plane of the drawing. The focussing electrodes ofthe first pair are disposed opposite each other on the two sides of theion path 28 and consequently act upon the thickness ofthe beam whereasthe second pair of focussing electrodes 25, 26 are disposed oppositeeach other at right angles to the first pair and work on the edges ofthe beam. The electrode pair 22, 23 is lmore negative than the electrodeI8. Similarly the second pair of focussing electrodes 25, 26 is morenegative than the first; pair while the electrode I9 having the slit Szis-'the most negative of all and conveniently may be connected toground.

The two chambers of the head are enclosed within an envelope 30 which isevacuated by conventional means such as a pump (not. shown) For thecurrent supply for the head, a battery 32 or other constant voltagesupply is provided. This voltage supply is connected to the ends of apotentiometer 34. The positive end of this potentiometer and its sliderare connected through a switch 36 to a condenser 38. A potential dividercircuit is connected across this condenser and takes the form of a4resistor 40 connected in series with a. first pair of poteniometers 42,44 and a second pair of potentiometers 46, 48. T-he potentiometers ofeach pair are connected in parallel and the second pair ofpotentiometers is connected through a common point in series with aresistor 50 to ground. The negative end of the potential dividingnetwork is connected not only to ground but also to the second or outletelectrode S2. The inlet electrode I8` having the slit Si' is connectedto the positive side of the potential dividing network, that is to theupper end of the resistor 40 as shown in the drawing. The first pair ofauxiliary or focussing electrodes 22, 23 are connected respectively tothe sliders of the potentiometers 44, 42 and the second pair ofauxiliary or focussing electrodes 25, 26 are connected respectively tothe sliders of the potentiometers 48, 46. In this fashion, the electrodeI8, the first pair of focussing electrodes 22, 23, the second pair offocussing electrodes 25, 26 and the outlet electrode I9 are increasinglymore negative so that ions formed at the gap between the two chambers byelectron bombardment of the gas molecules are attracted into andpropelled through the second chamber. By adjusting the settings of thetwo pairs of potentiometers, one or more transverse voltage componentsmay be set up within the second chamber to change the focussing of theion beam therein.

In the operation of the apparatus illustrated, a sample of gas isadmitted into the first cham- |ber under low pressure and molecules, ofthis vsample eventually arrive in the neighborhood of the outlet I2. Asthe molecules escape from the outlet I2, they `are bombarded byelectrons and become ionized and the electrical field created by theelectrode I8 and the other more negative electrodes within the secondchamber tends to attract ther ions and propels them through the secondchamber toward the analyzer. The movement of the ions into the secondchamber is aided by a iield leakage which is emphasized by theconductive walls of the first chamber in the neigh borhood of the outletI2.

When the ion beam has been formed and focussed by proper'adjustment ofthe potentiometers, the analysis proceeds. For example, the condenser 38may be charged from the power source 32. When the switch 36 is opened,the condenser is discharged through the potential dividing network, sothat there is a gradual de crease in voltage in all electrodes of thesecond chamber. As the voltage decreases, the path of the beam lwithinthe analyzer tends to change so that divergent beams therein are sweptone after the other over a conventional collector (not shown); Thispermits the several ion beams to be discharged sucessively at thecollector. A galvanometer or other conventional means connected to thecollector thus indicates or records a series of ion currents, which isthe mass spectrum of the material undergoing analysis.

v Although the auxiliary electrodes are increasingly negative in theexample illustrated, they need not be, and in some cases it may bedesirable to have them, or some of them, more positive than the mainelectrodes.

anaees My invention permits the simplification oran ionization chamberfor a mass spectrometer, without reducing the stability or sensitivityof the instrument as a whole.

I claim:

1. In a mass spectrometer, the combination which comprises a iirstchamber having a constricted outlet and an inlet for molecules to beionized, the wall of the chamber at least in the neighborhood of theoutlet being of electrically conductive material, a second chamberhaving a constricted inlet adjacent to and substantially coaxial withthe outlet but separated therefrom by a small gap, the wall of thesecond chamber at least in the neghborhood of the inlet thereto being bfconductive material, means for projecting an electron beam across thegap transverse to the axis of the inlet of the second chamber andtheoutlet ofthe first chamber in contact with molecules passing out of thatoutlet, an electrode disposed in the second chamber, and means forestablishing an ion propelling potential in the second chamber betweenthe inlet of the, second chamberiand the electrode.

2. In a mass spectrometer. the combination..

which comprises a first chamberhaving a conto the axis of the inlet ofthe second chamber and the outlet of the first chamber in contact withmolecules passing out of that outlet, an electrode disposed in thesecond chamber, and means for establishing an ion propelling potentialin the second chamber between the inlet of the second chamber and theelectrode to form an ion beam therebetween, a set of auxiliaryelectrodes mounted opposite each other on the two sides of the ion beam,and means for impressing a, potential between the electrodes of the setto aect the focussing of the ion beam.

4. In a mass spectrometer, the combination which comprises a firstchamber having a constricted outlet and an' inlet for-molecules to beionized, the wall of the chamber at least in the neighborhood of theoutlet being of electrically v conductive material, a second chamberhaving a a small gap, the wall of the second chamber at stricted outletand an inlet for molecules to be ionized, the wall of the chamber atleast in the neighborhood of the outlet being of electrically conductivematerial, a second chamber having a constricted-inlet adjacent to andsubstantially coaxial with the outlet butv separated therefrom by asmall gap, the wall of the second chamber at least in the neighborhoodof the inlet thereto being of conductive material, means for projectingan electron beam across the gap transverse to the axis of the inlet ofthe second chamber and .the outlet of the ilrst chamber in contact withmolecules passing out of that outlet, an electrode disposed in thesecond chamber, and means for establishing an ion propelling potentialin the second chamber between the inlet or the second chamber and theelectrode to form an ion beam therebetween, an auxiliary electrodemounted on one side of the ion beam, and means for impressing apotentialon that electrode to laffect the focussing of the ion bearn.

3. In a mass spectrometer, vthe combination which comprises a firstchamber having a constricted outlet and an inlet for molecules to beionized, the wall of the chamber at least in the neighborhood of theoutlet being of electrically conductive material, a secondchamber havinga constricted inlet adjacent to and substantially coaxial with theoutlet but separated therefrom by a small gap, the wall of the secondchamber at least in the neighborhood of the inlet thereto being ofconductive material, means for projecting an electron beam across thegap transverse least in the neighborhood of the inlet thereto being ofconductive materiahmeans for projecting an electron beam across the gaptransverse to the axis of the inlet of the second chamber and the outletof the ilrst chamber in contact with molecules passing out `of thatoutlet, au

means for establishing `an ion propelling potential in the secondchamber between the inlet of the second chamber andfthe electrode, toform ,an ion beam therebetween, a plurality of sets of auxiliaryelectrodes, the electrodes of each set being mounted opposite each otheron opposite sides of the ion beam, and means for impressing potentialacross the ion beam between the electrodes of each set.'

5. In a mass spectrometer,l the combination which comprises a ilrstchamber having a constricted outlet and an. inlet for molecules to` beionized, the wall of the chamber at least in the neighborhood of theoutlet being of electrically. conductivematerial, a second chamberhaving a constricted inlet adjacent to and -substantially coaxial withthe outlet but separated therefrom by a small'gap, the wall of thesecond chamber at least in the neighborhood of ,the inlet thereto beingof conductive material, means for projecting an lectron beam across thegap transverse HAROLD W. WASHBURN.

